Ashot Chilingarian

Lightning initiation remains unresolved because electric fields measured in thunderclouds are well below the conventional air breakdown field. Relativistic runaway electron avalanches (RREAs) provide a possible preconditioning mechanism because their threshold is roughly an order of magnitude lower and because they produce intense ionization and thunderstorm ground enhancements (TGEs). Examination of the largest TGEs reveals a systematic temporal ordering: lightning is not observed during the growth of the particle enhancement or at its peak, but associated flashes occur preferentially during the declining phase or at the end of the TGE, frequently terminating the remaining particle flux. Together with the strong spatial concentration of lightning around TGE-producing storms, this temporal asymmetry indicates that lightning occurrence is not controlled simply by the instantaneous maximum of the atmospheric electric field. We interpret these observations as evidence that sustained RREA ionization progressively preconditions the strong-field region, facilitating subsequent streamer and leader development; the resulting discharge then reorganizes the cloud charge and terminates the accelerator. Thus, RREAs are not merely a radiation signature of strong thunderstorm electric fields but contribute causally to lightning initiation.

Aims . The Large High Altitude Air Shower Observatory (LHAASO) Collaboration has recently reported the first detection at TeV energies of a low-luminosity radio galaxy, NGC 4278. The aim of this work is to investigate the high-energy properties of NGC 4278 during the flaring and subsequent quasi-quiescent states with the Florian Goebel Major Atmospheric Gamma Imaging Cherenkov (MAGIC) telescopes. Methods . NGC 4278 is located in the field of view of two blazars, 1ES 1215+303 and 1ES 1218+304, previously observed by the MAGIC telescopes. Therefore, we reanalyzed MAGIC observations made between 2010 and 2024 on these sources. We also modeled the broadband spectral energy distribution of the source during and after the flaring state at TeV energies. Results . We did not detect any statistically significant γ-ray emission from NGC 4278 with MAGIC. The corresponding upper limits obtained using the entire MAGIC dataset ( F _{UL,>150 GeV} = 1.5 × 10 ⁻¹² ph s ⁻¹ cm ⁻² ) are consistent with the LHAASO results. The best-fit models obtained for both emission states suggest that the emitting region is strongly particle-dominated, and an efficient acceleration mechanism has to be in action in order to reach TeV energies. The transition between the flaring and quasi-quiescent state cannot be explained by a simple radiative cooling of the emitting particles. The inferred jet power, on the order of L _jet ∼ 10 ⁴² erg s ⁻¹ , is dominated by the kinetic component in both states and it is in a good agreement with previous, time-averaged observational estimates, supporting the idea that such high-energy flares might be recurrent. The jet, however, remains too weak to break the host-galaxy confinement.

The relationship between atmospheric electric field (AEF) strength and observable gamma-ray production in the upper thundercloud dipole is not monotonic. Using upward-directed CORSIKA simulations with 1–100 MeV seed electrons, we investigated the development of relativistic runaway electron avalanches (RREAs) in acceleration regions of thicknesses 1, 1.5, and 2 km, spanning altitudes from 7.3 to 15.3 km. For all acceleration lengths, the escaping gamma-ray yield first increases with AEF, reaches an altitude-dependent maximum, and then decreases as the field is further increased. The ratio of escaping gamma rays to runaway electrons decreases with altitude, indicating a progressive reduction in bremsstrahlung conversion efficiency in rarefied air. These results show that upper-dipole RREA simulations should employ altitude- and geometry-dependent electric fields rather than simply the largest physically possible field.

The relationship between atmospheric electric field (AEF) strength and observable gamma-ray production in the upper thundercloud dipole is not monotonic. Using upward-directed CORSIKA simulations with 1–100 MeV seed electrons, we investigated the development of relativistic runaway electron avalanches (RREAs) in acceleration regions of thicknesses 1, 1.5, and 2 km, spanning altitudes from 7.3 to 15.3 km. For all acceleration lengths, the escaping gamma-ray yield first increases with AEF, reaches an altitude-dependent maximum, and then decreases as the field is further increased. The ratio of escaping gamma rays to runaway electrons decreases with altitude, indicating a progressive reduction in bremsstrahlung conversion efficiency in rarefied air. These results show that upper-dipole RREA simulations should employ altitude- and geometry-dependent electric fields rather than simply the largest physically possible field.

Ground Level Enhancements (GLEs) are rare solar energetic particle events in which relativistic solar protons produce measurable increases in secondary neutrons and muons at the ground level. For space weather operations, the key real‐time measurement is the spectral hardness of the solar proton population. This indicates that the SEP protons extend far enough to include substantial fluxes above 50 MeV, posing risks to satellite electronics, electrical lines, and pipelines on Earth. We analyze GLE 77 (11 November 2025) using measurements from Aragats (3,200 m) at the Aragats Solar Neutron Telescope (ASNT) and the SEVAN Light spectrometric detector. We employ energy‐deposit histograms of simultaneously detected muon‐ and neutron‐enriched fluxes to recover differential energy spectra and derive the muon‐to‐neutron ratio. The spectra are compared with a unified forward‐modeling chain (CORSIKA atmospheric cascades and GEANT4 detector response) processed through the same reconstruction procedure as the data. Both instruments reveal a two‐peak temporal structure that coincides with neutron monitor observations. The observations are most consistent with a dominant contribution from primary protons near 9–10 GeV, that is, near and slightly above the local geomagnetic cutoff. These neutron–muon spectrometric diagnostics offer a practical method for near‐real‐time tracking of proton spectral hardness during future GLEs.